Purpose: Type 2 diabetes increases the predisposition to atherosclerotic cardiovascular diseases. Therapies activating glucagon-like peptide-1 (GLP-1) signalling are increasingly used in the treatment of diabetes. These therapies have influence on cardiovascular function, but the expression of GLP-1 receptor (GLP-1R) in the atherosclerotic arteries is unclear. We evaluated the level of GLP-1R expression in the aorta of atherosclerotic and diabetic mice by immunohistochemistry and a novel GLP-1R targeting PET tracer ⁶⁸Ga-NODAGA-Exendin-4.

Methods: Eight atherosclerotic low density lipoprotein receptor deficient mice expressing only apolipoprotein B100 (LDLR^-/-ApoB^100/100), seven atherosclerotic and diabetic mice overexpressing insulin-like growth factor II (IGF-II/LDLR^-/-ApoB^100/100) and six healthy C57BL/6N mice were studied. Mice were fasted for 4 hours and glucose tolerance test was performed. The ⁶⁸Ga-NODAGA-Exendin-4 (20 ± 2 MBq) was intravenously injected and at 60 minutes post-injection, aorta and other tissues were excised and measured for total radioactivity by using a gamma counter. The aorta was then cut into serial longitudinal cryosections for autoradiography, histology and immunohistochemistry of GLP-1R.

Results: Aortas of LDLR^-/-ApoB^100/100 and IGF-II/LDLR^-/-ApoB^100/100 mice contained advanced atherosclerotic plaques. Diabetic mice demonstrated hyperglycemia and glucose intolerance. Immunohistochemistry showed GLP-1R expressing cells in atherosclerotic plaques. Compared with healthy mice, ex vivo gammacounting revealed increased (p < 0.01) uptake of ⁶⁸Ga-NODAGA-Exendin-4 in aortas of both LDLR^-/-ApoB^100/100 and diabetic IGF-II/LDLR^-/-ApoB^100/100 mice (0.38 ± 0.09 vs. 0.82 ± 0.32 and 0.73 ± 0.16 % IA/g, respectively). Autoradiography showed focally increased (p < 0.001) tracer uptake in atherosclerotic plaques compared with healthy vessel wall of the same mice (plaque-to-vessel wall ratio 1.6 ± 0.1). ⁶⁸Ga-NODAGA-Exendin-4 uptake was comparable in diabetic and non-diabetic mice, but the uptake in atherosclerotic plaques of diabetic mice correlated with the blood glucose levels (r = 0.89, p = 0.008). Tracer uptake in plaques was blocked with an excess amount of unlabelled peptide.  

Conclusions: ⁶⁸Ga-NODAGA-Exendin-4 and immunohistochemistry detected increased GLP-1R expression in the atherosclerotic aorta of hypercholesterolemic mice irrespective of the presence of type 2 diabetes. Imaging of GLP-1R may help to study the functions and potential of GLP-1 based therapies in type 2 diabetes-related atherosclerosis.